The term "nuclear fuel element" as used herein is intended to refer primarily to graphite or carbon-containing or coated bodies which contain nuclear fuel materials which have at least partly undergone a fission reaction, although the invention may be applicable to other nuclear fuel bodies or elements as well. Generally the invention is intended to apply to spent fuel elements, i.e. fuel elements which have been removed from a nuclear reactor core because at least part of fissionable material contained therein has undergone the fission reactor.
It is known that nuclear fuel elements containing fissionable fuel materials undergo during their period within the reactor core only a relatively small amount of decay, fission or consumption, i.e. only a small amount of the potentially fissionable material participates in the fission reaction.
During the fission reaction, fission products are formed which are considered neutron poisons, i.e. reduce the neutron generation capability or reactivity of the fuel element. When the activity of the fuel element is diminished to a certain degree, it is common practice to remove the fuel elements and replace them by fresh fuel, the removed fuel elements being referred to as a spent fuel.
In general, only about 3 to 4% by weight of the potentially fissionable material in the fuel element can be considered to be consumed before the fuel element is deemed to be spent. The greater part of the fissionable fuel remains unreacted and thus unused so that it can be recovered by nuclear fuel processing procedures well known in the art.
Because of the high level of reactivity, spent fuel elements after removal from the reactor core are stored in radiation-shielding storage vessels to allow radioactivity to decay sufficiently to permit reprocessing. The storage must be effected under conditions such that criticality is not reached in the storage vessel, i.e. under conditions such that the self-sustaining chain reaction which results from criticality does not occur. This is achieved by an appropriate geometric arrangement of the fuel elements.
The storage vessels for this type of activity-reducing decay of spent fuel elements just removed from a nuclear reactor is generally effected within the nuclear reactor installation itself.
After a satisfactory degree of decay, the spent fuel elements are amenable to reprocessing which is generally effected at a facility other than the nuclear reactor and may be transported and stored in appropriate transport receptacles. Naturally, during transport, care must be taken to avoid any release of the radioactive material into the environment by accident or otherwise.
The initial decay storage of the spent nuclear fuel elements upon their removal from the reactor core generally utilized water tanks into which the nuclear fuel elements were dumped or lowered with appropriate care so that criticality would not be reached.
For both the initial decay period and for subsequent storage and transport, however, there remains the need for a technique which will prevent or diminish the dangers represented by the activity of spent nuclear fuel elements, both with respect to reaching criticality and with respect to release of radioactivity into the environment.